Instrument and method for 3-dimensional atomic arrangement observation

ABSTRACT

3-dimensional observation is carried out on the atomic arrangement and atomic species in a thin-film specimen at an atomic level in order to clarify the existence states of defects and impure atoms in the crystals. For that purposes, the present invention provides an instrument and a method for 3-dimensional observation of an atomic arrangement which are implemented by a system comprising a scanning transmission electron microscope equipped with a field emission electron gun operated at an acceleration voltage of greater than 200 kV, a specimen goniometer/tilting system having a control capability of the nanometer order, a multi-channel electron detector and a computer for executing software for controlling these components and 3-dimensional image-processing software. Point defects and impure atoms, which exist in joint interfaces and contacts in a ULSI device, can thereby be observed. As a result, the causes of bad devices such as current leak and poor voltage resistance can be analyzed at a high accuracy.

This is a divisional of application Ser. No. 07/882,970, filed May 14,1992, now U.S. Pat. No. 5,278,408.

BACKGROUND OF THE INVENTION

The present invention relates to an instrument and method for theobservation of point defects, impure atoms and their clusters whichexist at joint interfaces and contacts in an integrated device formedinto a layered structure such as a memory or fast-calculation device.

As described in Proc. Mat. Res. Soc. Symp. Vol. 183 (Materials ResearchSociety, San Francisco, 1990) p. 55, the conventional electronmicroscope can be used for inferring a 3-dimensional atomic arrangementfrom several electron microscope images observed from differentdirections. In addition, a technique for obtaining a 2-dimensional imageof a 3-dimensional atomic structure is disclosed in Japanese PatentLaid-open No. 61-78041.

SUMMARY OF THE INVENTION

With the conventional techniques mentioned above, it is necessary toprepare a large number of thinned pieces having a thickness of the orderof several nm by cutting a specimen in various directions. In this case,if a target structure in the specimen has an infinitesimal size of theorder of nanometers, it is impossible to cut the structure into aplurality of pieces and, thus, impossible to carry out 3-dimensionalobservation. Even if the target structure is large enough to allow thethinned pieces to be prepared, only part of the target structure iscontained in such a piece so that a lot of information is found missingwhen constructing a 3-dimensional structure based on the electronmicroscope images of the pieces. In addition, since the observer has toinfer a 3-dimensional structure while taking the relation betweenobservation directions and their electron microscope images of thinnedpieces, the technique results in very inadequate precision. The accuracyof the observation directions is effected by errors in the angle settingwhen specimen pieces are cut out and inclinations of the specimen piecesset on the specimen holder of the electron microscope. It is difficultto make the observation conditions by electron microscopes completelyuniform for all the specimen pieces. The resulting errors thus give riseto variations in image contrast. An inference image formed by diffractedelectrons, or a lattice image, varies depending upon, among otherthings, the thickness of the specimen and electron diffractionconditions. In addition, even though information on the atomicarrangement can be obtained from a lattice image, it is difficult toidentify the atomic species of impurities and point defects.

In addition, it is disclosed in Japanese Patent Laid-open No. 61-78041that the electron incidence direction to the specimen surface is fixedand all reflected characteristic X-rays generated in the specimen can beobtained by changing the direction of detection. Information on thestructure of a 3-dimensional atomic arrangement close to the surface isthereby obtained. Nevertheless, the obtained information is limited toone to two atomic layers on the surface due to the use of all thereflected characteristic X-rays. In addition, since the characteristicX-rays are generated from a region of the micron order, it is impossibleto obtain high resolution at an atomic level. It is thus extremelydifficult to obtain a 3-dimensional atomic arrangement in the bulk witha high resolution at an atomic level.

It is an object of the present invention to obtain a 3-dimensionalatomic arrangement and atomic species in the bulk with a high resolutionat an atomic level using only a single thin-film specimen and, thus, toallow a 3-dimensional atomic structure to be analyzed accurately in ashort period of time.

In order to achieve the object described above, a system comprising ascanning transmission electron miroscope, a specimen goniometer/tiltingsystem, a multi-channel electron detector and a computer was built. Thescanning transmission electron microscope includes a unit for radiatingan electron beam having a diameter equal to or smaller than the size ofone to two atoms. The specimen goniometer/tilting system can becontrolled to move a specimen by a distance of the order of nanometers.The multi-channel electron detector allows the range of detection anglesof scattered electrons to be arbitrarily set. The computer is used forexecuting software for controlling the electron microscope and softwaresfor image processing. The system is thus equipped with facilities forobserving a 3-dimensional structure. To speak in more concrete terms,the system is characterized in that some projection images of atomicarrangement are obtained within an angular increment range θ from apredetermined inclination angle. While rotating the specimen over anangle in a range smaller than the angular increment θ, n images of2-dimesional atomic-arrangement are produced. Note that within theangular range in which the projection image of atomic arrangement isobtained, the so-called channeling phenomenon must occur at least once.In addition, the angular increment θ is equal to tan⁻¹ (d/t), where d isthe distance from an atom to an adjacent one in the specimen and t isthe thickness of the specimen. From the n images of 2-dimensionalatomic-arrangement obtained as such, atomic coordinates with roughprecision and atomic species are identified. Next, a 2-dimensionalatomic-arrangement image is simulated by the informations.

The simulated image is then compared to the 2-dimensionalatomic-arrangement images actually measured. Atomic coordinates andatomic species with high accuracy are obtained as both the images matcheach other. The accurate atomic coordinates and atomic species are usedto display a 3-dimensional atomic-arrangement image.

Accordingly, not only is a 3-dimensional atomic arrangement observed,but a structural analysis can also be performed as well using the samesystem.

A thin-film specimen is observed using the scanning transmissionelectron microscope using an electron beam with a diameter equal to orsmaller than the size of one to two atoms. The observation can result inan atomic-arrangement image. By observing the specimen while varying itsinclination by means of the specimen goniometer/tilting system,atomic-arrangement images from various directions can be obtained. Byapplying image processing to the atomic-arrangement images obtained forvarious inclination angles, a 3-dimensional atomic arrangement of thespecimen can be constructed and atomic species can be identified from ananalysis of a relation between the detection-angle ranges of scatteredelectrons used in the imaging and the degrees of the image contrast.

BRIEF DESCRIPTION OF THE DRAWINGS

(FIG. 1 is an explanatory view showing the principle of image formationusing an electron beam with a diameter equal to or smaller than the sizeof one to two atoms.) FIG. 1(a) shows states of transmission andscattering electron beams and an electron-microscope image when electronbeam is parallel to the direction of atomic columns. FIG. 1(b) showsstates of transmission and scattering electron beams and anelectron-microscope image when electron beam has an incident angle θ tothe direction of the atomic columns.

FIG. 2 is an explanatory view showing relations between the scatteringelectron intensity and the scattering angle for atoms with low and highatomic numbers.

FIG. 3 is an explanatory view for measurement of scattering electrons bya multi-channel electron detector in a scattering angle between γ and δ.

FIG. 4 is an explanatory view showing a process of constructing a3-dimensional atomic structure by image processing of 2-dimensionalatomic images observed at a variety of inclination angles θn of aspecimen.

FIG. 5 is a diagram showing an overall structure of an embodimentaccording to the present invention.

FIG. 6 is an explanatory diagram showing a relation between the angularincrement (θ) of the specimen, the distance from an atom to an adjacentone (d) and the thickness of the specimen (t).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the diagrams described briefly above, embodiments accordingto the present invention are explained as follows. FIG. 5 is a diagramshowing a basic configuration of an electron-microscope apparatus usedin the embodiments according to the present invention. As shown in thefigure, the apparatus comprises a field emission electron gun 8,condenser lenses 9, electron deflector coils 10, object lenses 11, aspecimen goniometer/tilting system 12, an electron detector 13, acomputer 14 for executing control and image-processing software, anX-ray detector 15, an energy analyzer 16, a specimen preparation room 17and a specimen transfer system 18. In order to generate an electron beamwith a diameter equal to or smaller than the size of one to two atoms,an acceleration voltage of at least 200 kV is applied to the fieldemission electron gun 8 and magnetic lenses for illumination with smallaberration are employed. A specimen 19 is scanned by the beamdeflecting/scanning coil 7 by applying an electron beam to the specimen19. The electron detector 13 has a multi-channel typed matrix of aplurality of photosensitive devices. The intensities of electronsscattered and transmitted by the specimen 19 can be measured byidentifying relations between the addresses of the photosensitivedevices in the matrix and the scattering angles and directions of theelectrons. Even though CCD photosensitive devices are typically employedin the electron detector 13, photosensitive devices of other types withhigh sensitivity can also be used as well. The specimengoniometer/tilting system 12 comprises a step motor and a goniometerwhich are controlled by the computer 14. This allows the inclination ofthe specimen 19 to be adjusted in the milliradian order. So, thepositional aberration is compensated in the nanometer order. Thecomputer 14 executes the control and image-processing software, allowingintensities and distribution of electrons measured by the electrondetector 13 to be input and stored into memory in synchronization withthe scanning operation of the incident electron beam. In addition, thecomputer 14 is also capable of carrying out a variety of imageprocessings.

Next, a step of observing a 3-dimensional atomic arrangement accordingto the present invention is described. FIG. 1 shows interaction betweenan atom 2 constituting the thin-film specimen 19 and incident anelectron beam 1 having a diameter equal to or smaller than the size ofone to two atoms. FIG. 1(a) shows a case in which the incident electronbeam 1 is parallel to the direction of the atomic columns of thethin-film specimen 19. In this case, an electron incident beam 1 betweentwo adjacent atomic columns is transmitted through by a channellingphenomenon without being scattered by the atoms 2. Note that thechannelling phenomenon is a phenomenon in which an electron beam 1 ispassed through. An incident electron beam 1 hitting an atomic column isscattered by the first atom 2 on the atomic column. By measuring theintensity of a transmitted or scattered electron 4 or 5 insynchronization with the scanning operation of the incident electronbeam 1 by means of the electron detector 13, a projection image ofatomic arrangement 6 can thus be observed. Next, the thin-film specimen19 is inclined to form an angle θ with the incident electron beams 1. Asshown in FIG. 6, the angle θ is set to a value smaller than tan⁻¹ (d/t),where d is the distance from an atom to an adjacent one on the thin-filmspecimen 19 and t is the thickness of the thin-film specimen 19. Thoughthe gap between two adjacent atomic columns as seen from the incidentdirection of the electron beams 1 becomes smaller, a channellingelectron exists. As shown in FIG. 1(b), the projection image of thearrangement 6 corresponds to a projection image viewed from an inclineddirection forming the angle θ with the atomic columns. In this case, theview of an impure atom 3 is different from that of FIG. 1(a ). That isto say, the impure atom 3 in FIG. 1(a) is not visible because it isshadowed by an atom 2 located right above it. In the case shown in FIG.1(b), however, the different atom 3 is visible. Accordingly, theincident electron beam 1 is scattered also by the impure atom 3. Ingeneral, relations between the scattering angle and the intensity of ascattered electron are shown in FIG. 2. As shown in the figure, thescattered electron intensity is distributed among the scattering angleswith a peak located at certain scattering-angle values. The distributioncurves are flatter for high scattering-angle values. The distributioncurves are also different from each other depending upon the atomicnumber (Z). The larger the value of the atomic number (Z), the more thedistribution curve is shifted to the side of large scattering-anglevalues. Accordingly, a scattering angle β for the peak intensity ofelectrons 5 scattered by the impure atom 3 is different from ascattering angle α for the peak intensity of electrons scattered by asurrounding atom 2. In this case, the atom 2 has a greater atomic numberthan the impure atom 3. Taking the distribution shown in FIG. 2 intoconsideration, the detection angle range of the scattered electrons 5used in the imaging by the electron detector 13 is set between angles γand δ shown in the figure. FIG. 3 shows a state of operation of theelectron detector 13 for the detection angle range between γ and δ. Asshown in the figure, the electron detector 13 has a multi-channel matrixconfiguration which comprises a plurality of photosensitive devices 7,each photosensitive device has a size of several micrometers or less.

When the incident electron beam 1 hits the specimen 19, electrons 5 arescattered at a variety of scattering angles, arriving at the electrondetector 13. Only electrons with scattering angles between γ and δ areused for creating a projection image of atomic arrangement 6. That is tosay, only the intensities of scattered electrons 5, which are detectedby photosensitive devices 7 located between two concentric circlescorresponding to the scattering angles γ and δ, are measured insynchronization with the scanning operation of the incident electronbeam 1. The range of detection angles is set by specifying the addressesof the photosensitive devices 7 with the computer 14. With suchmeasurement, the difference in contrast between atoms on the projectionimage can be recognized. In this case, the atom 2 is bright whereas thedifferent atom 3 is dark. By embracing the same principle, thedifference can still be recognized even if a vacancy exists at theposition of the impure atom 3. Information on distribution of scatteredelectron intensities for various atoms are stored in the computer 14.Accordingly, the detection angle ranges for the various atoms can be setin the electron detector 13. The various atoms can thus be distinguishedfrom each other based on differences in image contrast between them. Inaddition, since the specimen goniometer/tilting system 12 allows theinclination angle of the specimen 19 to be controlled in the milliradianorder, an inclination angle can be set at the condition of thechannelling-phenomenon. Moreover, the position of the specimen 19 can becontrolled using the computer 14 so that the target of observation onthe specimen 19 is always located at the center of the observation area.The computer-based control is carried out by finding the amount ofaberration in the position of the specimen 19, that results with thespecimen 19 inclined, using the image processing. By continuouslyobserving images while varying the inclination angle and storing imagedata in the computer 14, the projection images of atomic arrangement 6observed from a variety of directions can be obtained.

The image processing constructs a 3-dimensional structure of the atomicarrangement based on projection images of atomic arrangement 6 (I₁, I₂to I_(n)) obtained at inclination angles (θ₁, θ₂ to θ_(n)) respectivelywith a procedure shown in FIG. 4. The 3-dimensional structure of theatomic arrangement is displayed on a CRT of the computer 14. On theprocedure, at first, 3-dimensional image processing is performed on theprojection images of atomic arrangement 6 (I₁, I₂ to I_(n)) to identify3-dimensional coordinates, the symmetry, and the regularity of theatoms. The atomic arrangement identified above are then combined withmeasurement data of atomic species to determine a 3-dimensionalstructure of the atomic arrangement of the specimen 19. The techniqueadopted for constructing the 3-dimensional structure is the same as thatdescribed on Page 15 of No. 6, Vol. 17, 1978 of Measurement and Control,a technical journal. The image processing software for constructing the3-dimesional structure, which is capable of creating a 3-dimensionalconfiguration based upon information obtained even from a range ofpossible inclination angles 0 to about 20 degrees of a specimen. Forexample, the softwares are Fourier deconvolution method and the seriesexpansion method. The image processing software is executed by thecomputer 14 which selects one of the techniques in accordance with theamount of information to be processed. Based on data of the 3-dimesionalstructure of the atomic arrangement, a projection image of the atomicarrangement 6 is then simulated. Software used in the simulation appliesa typical method such as the multi-slice technique. The simulated imageis then compared to the observed image in order to confirm whether ornot a projection image of the atomic arrangement 6 can be reproducedfrom the constructed 3-dimensional structure of the atomic arrangement.If the reproduction is impossible, the data of the 3-dimesionalstructure of the atomic arrangement is corrected to give anothersimulated projection image of the atomic arrangement 6. This operationis repeated until the simulated image matches the observed one. In thisway, the accuracy of the 3-dimensional structure of the atomicarrangement can be enhanced. The 3-dimensional structure of the atomicarrangement determined as such is finally displayed on the CRT of thecomputer 14 as a squint image or a cross-sectional view seen from anydesired direction.

The composition and bonding state of elements constituting the specimen19 can be analyzed by measurement of a characteristic X-ray by the X-raydetector 15 and measurement of loss energy of transmitted electrons bythe energy analyzer 16. A scanning tunnelling microscope is installed atthe specimen preparation room 17 in which the thinning process of thespecimen 19 is carried out by utilizing a field-evaporation effect thatoccurs when a field is applied to an area between a tip and the specimen19. In this way, atoms are stripped off one by one. Accordingly, thethickness of the specimen 19 can be controlled in atomic-layer orderwithout damaging the specimen 19 at all. By carrying out the operationto strip off atoms as such while observing the specimen 19 through thescanning tunnelling microscope, the structure of an infinitesimalportion of interest can be surely converted into a thin film with anaccuracy at the atomic level. Since the thin-film specimen 19 isconveyed by the specimen transfer system 18 to a specimen observationroom through a vacuum, the specimen 19 is neither contaminated noroxidized. In the specimen preparation room 17, the specimen 19 canundergo manufacturing and fabrication processes such as the specimencleaning and alteration using ion radiation and heating and thethin-film formation using evaporation and sputtering. Therefore, atomicstructures in a variety of states can be observed. Furthermore, thespecimen preparation room 17 can be removed from the electron microscopeand connected to the actual thin-film equipment used in thesemiconductor process. In such an arrangement, a specimen formed by thethin-film equipment is conveyed to the apparatus provided by the presentinvention in which the evaluation of its process conditions can becarried out.

As described above, the present invention allows the observation of the3-dimensional atomic arrangement at a high resolution of higher than 0.2nm The present invention also allows the analysis of atomic species. Inaddition, the present invention allows the composition and the bondingstate to be measured as well. Point defects, impure atoms and theirclusters which are difficult to examine using the conventional electronmicroscope can thereby be observed at a single-atomic level.Accordingly, the causes of ULSI devices' defects, thin film's formationconditions and the like can be evaluated at high accuracy. In the caseof the conventional electron-microscope techniques, as many specimensamples as numerous observation directions have to be prepared in orderto accomplish 3-dimensional observation. With the present invention,however, only a single specimen is required. As a result, the T. A. T.(turn-around time) of the evaluation process is substantially reduced ascompared to that of the conventional techniques.

What is claimed is:
 1. An instrument for 3-dimensional atomicarrangement observation comprising an electron gun, magnetic lenses forillumination, electron deflector coils and a specimen holder, saidinstrument comprising:a specimen goniometer/tilting system having adriving means connected with said specimen holder; an electron detectorfor observing a plurality of 2-dimensional atomic-arrangement images ofa specimen upon application of a scanning electron beam with anatomic-level diameter; means for compensating automatically a shift ofspecimen position induced by the specimen tilting; and a computer forexecuting control of said specimen goniometer/tilting system, saidelectron detector and said compensating means.
 2. An instrument for3-dimensional atomic arrangement observation according to claim 1,wherein said specimen goniometer/tilting system connected to saidcomputer is capable of fine-controlling the inclination of said specimenby finding an amount of positional aberration due to said specimeninclination so that a structure of interest in said specimen is alwaysheld at the center of an observation area.
 3. An instrument for3-dimensional atomic arrangement observation according to claim 1,wherein said electron detector comprises a plurality of photosensitivedevices arranged in a matrix configuration so that a range of scatteringangles and scattering directions of scattered electrons used forformation of said 2-dimensional atomic-arrangement images being selectedarbitrarily.
 4. An instrument for 3-dimensional atomic arrangementobservation according to claim 3, wherein said photosensitive device hasa size of several micrometers or less.
 5. An instrument for3-dimensional atomic arrangement observation according to claim 1,wherein said computer comprises image-processing software for, whenexecuted by said computer, storing said 2-dimensional atomic-arrangementimages observed at each inclination angle of said specimen at an angularaccuracy corresponding to the atomic distance and constructing a3-dimensional atomic-arrangement structure using said 2-dimensionalatomic-arrangement images.
 6. An instrument for 3-dimensional atomicarrangement observation according to claim 5, wherein saidimage-processing software includes a program for, when executed by saidcomputer, reconstructing an image of a 3-dimensional structure from aplurality of 2-dimensional projection images obtained by viewing said3-dimensional structure from a variety of angles.
 7. An instrument for3-dimensional atomic arrangement observation according to claim 6,wherein said image-processing software includes a program for, whenexecuted by said computer, constructing a cross-sectional image at anycross section of a 3-dimensional structure and a program for, whenexecuted by said computer, constructing a squint view of a 3-dimensionalstructure as seen from any arbitrary direction.
 8. An instrument for3-dimensional atomic arrangement observation according to claim 1,wherein said computer comprises control software for, when executed bysaid computer, controlling said instrument.